Casing for a writing instrument



FIG. 1 is a front elevational view of a casing for a writing instrument, in an exploded condition, showing my new design;

FIG. 2 is a front elevational view of my design in a closed condition;

FIG. 3 is a right side elevational view;

FIG. 4 is a left side elevational view;

FIG. 5 is a rear elevational view;

FIG. 6 is a bottom plan view; and

FIG. 7 is a top plan view thereof. 

The ornamental design for a casing for a writing instrument, as shown. 